Q: "swear by forty"& "slab"(adj), 1831

[Joel S. Berson]

I don't get the meanings of "swear by forty"and "slab" (although I
can guess about the latter!) in the following quotation.  And where
should they appear in the OED?

"A captain of militia was in the habit of swearing 'by forty.'  He
had, like many other officers who commanded 'slab' companies, a
troublesome set of fellows to deal with."

GBooks has this in _The Casket. Flowers of Literature, Wit, &
Sentiment_, Philadelphia: Atkinson, No. 6, June 1831, p. 288 (957 in
PDF).  It is attributed to the New York Constellation.

The title page for this issue (909 in PDF) is _Atkinson's Casket.
Gems of Literature, Wit and Sentiment_.  Apparently it was considered
more valuable by Issue 6.

I do not find any other (useful)  instances of "swear by forty" in
Google Books or Web besides printings of the same article.  Except
perhaps Samantha Dias's use of "swear by forty swords" in a poem
dated Dec 5, 2011.

Joel

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